Differential voltage contact probes typically transition two uncontrolled transmission lines (i.e. the probe tips) into a more controlled structure such as controlled impedances on a printed circuit board (PCB) or coaxial cables. “Browser” type probes are typically hand-held and therefore have some compliance or “give” to ensure reliable contact and also adjustable span to accommodate probing of different physical geometries. Due to these multiple degrees of freedom a mechanism may be required to maintain the ground of the transmission line at this transition point. An ideal mechanism may have many possible jobs depending on the exact implementation of the probe system including: maintain good electrical contact through full span adjustment; maintain a low inductance ground path (to maximize electrical performance); minimize the loop area of the probe (thus maximizing electrical performance) be robust and not easily damaged by the user; allow for “z-axis” compliance, or compliance normal to the probe tips (and still maintain electrical contact); allow for a very large number of span adjustments; be easy to assemble; and have a low material cost. These probes are typically used with oscilloscopes.
Performing all of these jobs effectively with a single mechanism may be one of the primary challenges in designing and building a browser type probe.
Keysight Technologies Inc. (the present Assignee) currently offers various differential, hand-held browser probes. The N5382A probe has no ground mechanism since the PCB ground provides a low inductance ground connection, and the tip wires transition directly into the PCB with a tip network and then into controlled impedance transmission lines. The span adjustment on this browser probe relies on bending the spring wires. Another Keysight browser probe is the E2675A. This browser probe uses a rotation-based ground mechanism and has a very large loop area. Keysight also sells the N5445A, which is a high-performance browser probe. The ground mechanism on this browser uses two flat, sliding blades each of which is connected to one probe arm while sliding against the other probe arm as the span is adjusted. This results in a good low inductance ground and very small loop area. The blades are designed to be user-replaceable. However, at small span widths, the excess ground blade dangles in space and can affect use of the probe by impairing view of the device under test (DUT).
Lecroy sells a Dxx05 browser probe that uses a flex circuit ground: this ground mechanism has a limited span range and is fully exposed to the customer which makes it easily damaged. The variable ground span relies on the flexing of pc material so the slot between the tips has to be fairly long. This long slot increases the loop area of the tip network and causes excessive inductance. It is also not user-serviceable. Furthermore, since this is a flex circuit there is no variety of metal materials to choose from (e.g. only copper).
Tektronix's probe uses a wire loop for the ground mechanism which may provide good span range, but at small span ranges the bulk of the wire loop is un-utilized and cumbersome. Also, the wire geometry results in poor loop inductance which sacrifices performance. The probe is fully user-exposed and user-serviceable by necessity.
In view of various shortcomings of conventional contact probes, there may be a general need for improved approaches to maintain electrical contact through the entire span range, maintain a low inductance ground path, protect the probe from damage/abuse, allow for a large number of span adjustments/cycles, ease both manufacturing and assembly, reduce cost and/or fit a variety of probe geometries.